When charging the battery of an electric vehicle, it is necessary to adjust the charging current to an appropriate value at all times, based on the specifications of the battery and the current charge state of the battery. The electronics necessary for this can be selectively arranged on board the vehicle or in the charging station. For an inductive transfer of energy from the charging station to the vehicle, the charging station must offer the maximum possible charging power if all the charging electronics are arranged in the vehicle, and this power must then be stepped down in the vehicle to the instantaneously required value. This leads to relatively high power losses both on the primary side and on the secondary side, and requires a correspondingly robust and therefore expensive design of the secondary-side charging electronics.
To avoid this, the power offered on the primary side can be controlled on the basis of the secondary-side power requirements, but appropriate communication from the secondary side to the primary side is necessary. This communication can either be handled via a completely separate communications channel, for example a wireless connection, or a data signal can be modulated onto the transfer link provided for inductive energy transfer. Both solutions, however, require special communication hardware on the primary and secondary sides, which is connected with corresponding costs.
A traction battery charging system with inductive coupling is known from DE 696 02 739 T2, in which the charging station can be controlled from a vehicle-side battery charging control module. A data communication means, which can be a data cable, a fiber-optic conductor or a wireless transmission link, is provided for communication between this battery charging module and the charging station.
For an inductive non-contact power transfer means for small devices such as mobile telephones and electric toothbrushes, DE 101 58 794 B4 teaches the adjustment of the primary-side power on the basis of the secondary-side power requirements by means of a primary-side control unit, which controls the power transfer by varying the duty ratio in the case of a low secondary-side power requirement, and by varying the frequency of a primary-side voltage-controlled oscillator in the case of a high secondary-side power requirement.